In numerous applications, like in drive trains with a converter-controlled electric machine, a current between an energy source, like an accumulator or a battery, and a load needs to be controlled by a switch. This type of switch is commonly referred to as a main switch or a main switching module. It is commonly known to implement a main switching module as a relay, which is an electromagnetically operated mechanical switch.
There are some requirements for main switching modules, particularly when applied in power circuits: (a) while in normal operating condition, a main switching module is expected to provide low-loss operation, even at high currents; (b) a main switching module must allow for safe current interruption, i.e., safe overload disconnection or short circuit disconnection; (c) when the load includes a converter, a DC-link capacitance must be charged when switching on the power circuit or the drive train, respectively. The main switching module should be configured to pre-charge the DC-link capacitance prior to normal operation (normal or perpetual operation without malfunction) of the drive train.
Relays, irrespective of whether they are applied in low-power or high-power applications, have several drawbacks. Relays, as being electromechanical switches, include a moving portion having an inherent inertia. This inherent inertia causes a delay between the time when a switching command is applied to the relays and the time when the relays actually switches. When a short-circuit occurs in the load, a significant increase of a short-circuit current may occur during a delay time between an instant when the short-circuit is detected and a switching command is generated and the instant when the relays switches. However, there are applications in which a delayed interruption of the short-circuit current may be hazardous.
Another aspect about short-circuit-save relays (or contactors) is that additional components have to be provided in order to make them electric-arc save, rendering those relays highly expensive, heavy, and considerably bulky.